Essentially a very small CRT, the E1T emits a ribbon-shaped electron
beam which is guided to the fluorescent screen. In operation, the beam
is moved one step at a time to the left by 'D' and moves to the next slot
in G4. At each slot, electrons will also pass through the corresponding
slot in A2 and on to the screen. When the beam reaches the far left it
strikes A1 which can be used to reset the beam and thus the count via
external circuitry.